1. Field of the Invention
The present invention relates to a process for producing silicon-containing isocyanate compounds by the thermally induced decomposition of urethane compounds which are carbamic esters containing silicon and at least one alkoxysilyl group or one siloxane unit.
2. Description of the Related Art
An isocyanate group has a high reactivity and reacts with an organic functional group having an active hydrogen such as a hydroxyl group, a primary or secondary amino group or a carboxylic acid group to form a urethane bond or a urea bond. Therefore, an organic polysiloxane having an isocyanate group introduced into the organic compounds is useful as a silicone modifier for organic compounds having an active hydrogen. Processes for efficiently producing the polysiloxanes containing an isocyanate group were proposed (see Japanese Patent Laid-Open No. 6-228161 (1994).
On the other hand, an alkoxysilyl group is also highly active, and polysiloxanes having such a functional group are useful as the silicone modifier. Further, it is known to use these polysiloxanes as a moisture curing agent, etc. Therefore, a silicon compound having an isocyanate group and at least one alkoxysilyl group in the organic compounds is highly valuable in the alkoxysilane modification of an active hydrogen-containing organic compound.
The isocyanate compounds are usually produced by phosgenation on an industrial scale. However, when a polysiloxane containing an amino group is reacted with phosgene, the intended polysiloxane-containing isocyanate compound cannot be obtained, since the siloxane bond is decomposed by hydrochloric acid formed as a by-product in the prior art. Also, when an alkoxysilylalkylamine having an alkoxy-silicon bond is reacted with phosgene, the bond between the alkoxy and silicon is decomposed by hydrochloric acid formed as a by-product. Additionally, an alcohol thus formed reacts with the isocyanate to lower the yield of the intended alkoxysilyl group-containing isocyanate compound, drastically. On the other hand, Japanese Patent Publication No. 5-8713 (1993) discloses a process wherein the reaction of alkoxysilylalkylamine with phosgene is carried out in the presence of a tertiary amine to neutralize hydrochloric acid formed as a by-product.
However, this phosgenation process has problems both in handling highly toxic phosgene and in the treatment of hydrochloric acid, which is formed as a by-product in the process and easily corrodes the apparatus. Under these circumstances, investigations were made for the purpose of developing an industrial process to be employed in place of the above-described process. For example, a process wherein a carbamatoorganosilane is gasified and then this compound is decomposed by heating in a gas phase is disclosed in Japanese Patent Laid-Open No. 7-258273 (1995). However, undesirable decomposition of this compound per se might occur in addition to the intended thermally induced decomposition, or side reactions such as the polymerization of the formed isocyanate might occur, since a considerably high heat is applied to the carbamatoorganosilane for the gasification thereof.
U.S. Pat. No. 3,607,901 discloses a process for producing silicon-containing isocyanate compounds. Although .gamma.-isocyanatopropyltrimethoxysilane is obtained by the thermally induced decomposition in Example 2 given in the specification, the yield of this product is only as low as 72.9%. It is pointed out that this process has another problem that since cyclic silyl carbamate formed as a by-product has a boiling point close to that of the intended product, the purification of the product is quite difficult.
Although various processes for producing isocyanate compounds containing silicon and an alkoxysilyl group were proposed as described above, these processes are those using highly toxic phosgene or vapor phase processes easily allowing undesirable side reactions in the gasification of the starting material. Thus, these known processes cannot provide the intended product with a high purity in a high yield and usually has economical advantages.